OK, here is CL-RemiMatrix, my library for #CommonLisp to interact with the #matrix protocol! This is just v0.1.0 and doesn't cover everything in the Matrix protocol, but it should be usable enough to write bots (I've done a few with it already) and maybe even a basic client. I'll of course keep adding to it as time goes on.

Source code repo, release tarbal, and so on are all here:
https://fossil.cyberia9.org/cl-remimatrix/

It uses my CL-RemiMarshal library internally for all its JSON (de)serialization to/from CLOS. Made it soooo easy to make CLOS classes for the protocol.